With global warming, governments around the world are becoming increasingly demanding on energy conservation and emission reduction. Therefore, finding new energy sources to replace fossil fuels has become an urgent need.
Solar energy is a clean, pollution-free and inexhaustible source of energy. At present, solar energy is utilized primarily via its conversion into electricity by solar panels. The electricity is then used to power electric water heaters, electric vehicles and satellite components.
A solar panel is a photovoltaic device generating electricity directly from light, particularly from sunlight. A solar panel of the prior art mainly comprises a back sheet, a photovoltaic circuit, an encapsulation material and a front sheet.
The encapsulation materials, such as polyethylene-vinyl acetate films, are used in solar panels to bond the front and back sheets together. In a lamination operation carried out at about 150° C., molten polyethylene-vinyl acetate flows into voids in a solar panel to encapsulate the solar cells. A conductive adhesive may also be used to connect solar cells to each other.
The primary role of a front sheet in a solar panel is to protect solar cells against mechanical impact and weathering. In order to make full use of the light, the front sheet must have a high light transmittance in a certain range of the spectrum (for example, for polycrystalline silicon solar cells, the range is 400-1,100 nm). The front sheet of a prior art solar panel is typically made of glass (usually low-iron tempered flint glass with a thickness of 3-4 mm) or polymeric materials.
The primary role of the back sheet in a solar panel is to protect solar cells and encapsulation materials and/or conductive adhesives against moisture and oxidation. During assembly of a solar panel, the back sheet is also used as a mechanical shield to prevent scratches and as an insulator. Therefore, excellent moisture barrier property and flexibility are required for the back sheet.
A back sheet commonly used in solar panels is a multi-layer adhesively laminated film, mainly including the following three types:
1) Laminated TPE film: fluoropolymers (e.g., polyfluoroethylene)/polyethylene terephthalate/ethylene-vinyl acetate copolymer containing 1%-70% vinyl acetate;
2) Laminated TPT film: fluoropolymer (e.g., polyfluoroethylene)/polyethylene terephthalate/fluoropolymer (e.g., polyfluoroethylene);
3) Laminated PET film: polyethylene terephthalate/polyethylene terephthalate/polyethylene terephthalate.
For instance, in order to form a low cost thermoplastic resin film that has excellent hydrolysis resistance and delamination resistance, and improves photoelectric conversion efficiency of solar cells by enhancing shielding capability, JP 2006-270025A disclosed a thermoplastic resin sheet for making back sheets of solar cells. The thermoplastic resin is a polycondensate of dicarboxylic acid derivatives and diol derivatives. The thermoplastic resin sheet provided by JP 2006-270025A includes, for example, polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, polyethylene 2,6-naphthalate, poly(1,4-cyclohexylene-dimethylene terephthalate), and polyethylene glycol-co-cyclohexane 1,4-dimethanol terephthalate. In particular, polyethylene terephthalate is widely used due to its low price. With environmental protection awareness becoming deeply rooted in the public mind and environmental regulations becoming more and more stringent, solar panels are being more and more commonly used to replace common coal- or gas-burning devices. Therefore, solar panels are also being more and more commonly used under a variety of harsh environmental conditions. This has resulted in new and higher requirements for solar panel performance, such as moisture resistance. Although conventional solar panel back sheets can provide good moisture barrier properties, there is still room for improvement.
In addition, during assembly of solar cells, component manufacturers often find that the flexibility and degree of warpage of the back sheets have a large impact on production. A back sheet that is too soft is not easily flattened. Wrinkles may occur during lamination. A back sheet that is too hard would warp to a large degree when bent under stress. Additional laminating machines are often required to flatten deformed back sheets in the production of solar cell components. This is quite time-consuming and adds to the cost. A back sheet made of laminated multi-layer PET film has insufficient flexibility and a higher degree of warpage. There is much room for improvement.
Of the above three types of back sheet structure, laminated TPE film and laminated TPT film have better mechanical strength and aging resistance, but have higher cost and higher degree of warpage, while laminated multi-layer PET film has lower cost, but lower flexibility and higher degree of warpage. The moisture barrier properties are yet to be improved for these three types of back sheets.
One objective of the present invention is to provide a laminated film that not only has excellent moisture barrier property and flexibility, but also has a lower degree of warpage in addition to a moderate price.
Another objective of the present invention is to provide a solar panel that utilizes the above-described laminated film.